Typically, in known position-measuring devices for sensing the position of two relatively movable objects, the position of a scanning unit relative to a scale is determined along at least one measurement direction. A measuring graduation is arranged on the scale along a graduation direction which corresponds to the measurement direction. The scanning unit and the scale are each connected to a respective one of the two movable objects. In known devices, the “sensitivity vector” of the position-measuring device, which denotes the respective effective measurement direction, is typically oriented parallel to the surface of the scale.
Also known are position-measuring devices whose sensitivity vector is oriented obliquely to the surface of a scale having a reflective measuring graduation. In this regard, reference is made, for example, to the Applicant's European Patent Application EP 1 762 828 A2. In a position-measuring device of this type, the inclined orientation of the sensitivity vector is ensured by an asymmetric configuration of the interferential scanning beam path. In such scanning beam paths, an incident beam is split into at least two sub-beams which are finally brought into interfering superposition. With such position-measuring devices, it is possible to obtain position information with respect to relative movement between the scanning unit and the scale both along a lateral measurement or displacement direction and along a vertical measurement or displacement direction. This means that such position-measuring devices can be used to measure changes in position along two degrees of freedom in translation. In such a position-measuring device, the path lengths of the interfering sub-beams are usually equal only at a certain nominal scanning distance between the scanning unit and the scale. If the scale or the scanning unit is moved out of the respective normal scanning distance, then the optical path lengths traveled by the interfering sub-beams will be different. Thus, a possible change in the wavelength of the light source used affects the phase of the interfering sub-beams, and thus also the determined position information. The scanning optical systems of such position-measuring devices are therefore referred to as chromatic or wavelength-dependent. Therefore, the light source used therein must have a sufficient coherence length and extremely low phase noise. To ensure this, such a light source must be stabilized in a complex fashion, which makes it correspondingly expensive.
The Applicant's German Patent Application DE 10 2015 203 188 A1 describes a further optical position-measuring device which is capable of acquiring position information with respect to relative movement between the scanning unit and the scale both along a lateral measurement or displacement direction and along a vertical measurement or displacement direction. Here, the scanning optical system is tilted by a certain tilt angle relative to the scale about an axis of rotation that is oriented parallel to the surface of the scale and extends perpendicular to the grating vector of the measuring graduation. In order to provide an asymmetric interferential scanning beam path configuration, provision is made not only to select a suitable tilt angle relative to the scale, but also to use, for signal generation, sub-beams resulting from non-symmetric diffraction orders at the measuring graduation such as, for example, the +3rd/−1st diffraction orders or the +1st/0th diffraction orders. However, the use of such diffraction orders is problematic in that the resulting signal intensity and/or mounting tolerance are/is too low.
FIG. 2 of U.S. Pat. No. 8,730,485 B2 illustrates another optical position-measuring device for measuring changes in position along two degrees of freedom in translation. This device employs a beam that is obliquely incident on the measuring graduation and uses the +1st/−1st diffraction orders at the measuring graduation for signal generation, thereby avoiding the problems of the aforementioned publication. However, the optical paths resulting for the sub-beams split at the measuring graduation differ in length between splitting and recombination. This in turn makes the position measurement dependent on wavelength fluctuations.